In medical diagnostic systems that use a radiation image conversion panel, the radiation image information about a subject such as the human body is first recorded on a stimulable phosphor and then either reproduced on a light-sensitive material (e.g. a photographic film) and the like or output as a visible image to a variety of display means.
The stimulable phosphor used in the medical diagnostic systems is a phosphor which, upon exposure to a radiation (e.g. X-rays, α-rays, γ-rays, electron beams or UV light), stores part of its energy and which, when later irradiated with exciting light such as visible light, emits a quantity of photostimulated luminescence that depends on the stored energy. For various reasons such as the ease of handling, the stimulable phosphor is typically used in the form of a panel comprising a support and a layer of the stimulable phosphor provided on it (namely, in the form of a radiation image conversion panel); the panel is commonly called an imaging plate (hereinafter sometimes abbreviated as IP).
The IP is used in a radiation image information recording and reading apparatus (which is hereunder referred to simply as an apparatus); this apparatus is typically an integral assemblage of an imaging unit which records radiation image information about a subject on the IP, a reading unit which applies exciting light to the IP having the radiation image information recorded thereon so as to photoelectrically read the radiation image information, and an erasing unit which removes the radiation image information that remains on the IP after the reading step.
The apparatus may adopt either a “cassette system” using the IP as it is accommodated in a cassette or a “built-in system” using the IP continuously in the system. The two systems are basically different in the number of IPs used: in the former system, a multiple of IPs are used sequentially whereas in the latter, only a few (mostly one or two) IPs are frequently used.
The essential part of a built-in apparatus may be found in JP 2000-122194 A; in FIGS. 2 and 3 accompanying that patent, a type of using two IPs is shown, and a type of using a single IP is shown in FIG. 4. Both types of apparatus are designed for surface reading, or reading information from the plane on which the radiation is incident.
JP 2002-285148 A gives a full account of the stimulable phosphor described above, and radiation image conversion panels (IPs as referred to in the specification of the subject application) that are fabricated by coating a variety of supports with that stimulable phosphor, as well as a method of recording radiation image using the panel. The structural design of the apparatus is not particularly specified in this patent.
When one attempts to improve the apparatus of the aforementioned built-in system by realizing smaller size, easy maintenance and convenient use, the following points should be considered.
For ultimate size reduction, it is preferred to use only a single IP. Needless to say, a design that satisfies this need repeats the cycle of the following steps: recording radiation image on a single IP, reading the information about the recorded radiation image, erasing the information about the radiation image that remains after the reading step, and using the IP again.
This design requires the IP to have a certain characteristic that may be called “suitability for repeated use”. This is none other than a characteristic of the stimulable phosphor used in the IP. To be more specific, when the apparatus is designed to use only one IP for the purpose of size reduction, one and the same IP shall be used repeatedly over a specified period of time, so it must be chosen to meet the requirement for sufficient durability.
The IP design factor that should be considered here is that it takes only a short time to erase the radiation image information on the IP after the reading step (this requirement is hereinafter described as “good erasure characteristic”). When one adopts the above-described process in which the cycle of recording radiation image information on a single IP, and reading and erasing the information is repeated within a short period of time, good erasure characteristic is important for the purpose of shortening the cycle time.
In previously filed and commonly assigned Japanese Patent Application No. 2004-81010 (JP 2005-266526 A) for “a radiation image information recording and reading apparatus”, is proposed an IP using a stimulable phosphor with good erasure characteristic, as well as a radiation image information recording and reading apparatus that employs the IP.
The stimulable phosphor used in that IP (and shown in the Examples) has the composition BaFBr0.85I0.15:Eu and, compared with two other stimulable phosphors in common use (BaFI:Eu and CsBr:Eu), this phosphor shows outstanding performance in such characteristics as the required quantity of erasing light and the required quantity of residual image erasing light (the “required quantity of residual image erasing light” means the luminescence measured in the same manner as in the quantity of erasing light after a predetermined period of time has passed from the light erasure).